1. FIELD OF THE INVENTION
This invention relates to a suspension control system for electronically limiting vibrations of a body of a motor vehicle in a vertical direction caused by resonance of the vehicle body when the vehicle runs and, more particularly, to a suspension control system of this kind having improved control reliability and improved durability.
2. DESCRIPTION OF THE RELATED ART
Electronic suspension control systems are known in which a fluid (pneumatic) cylinder mechanism is provided between a vehicle body and wheels, a control target (operating time) of the cylinder mechanism is set according to the magnitude of vibrations in a vertical direction acting on the vehicle body, and the vibration amplitude is limited on the basis of the control target.
In this kind of suspension control system, a vibration limiting control is started when the period of vertical vibrations of the vehicle body is within a predetermined range and when the amplitude is larger than a reference threshold value, and the vibration limiting control is stopped when the amplitude is reduced below a predetermined level such that the vibrations are sufficiently limited. During the vibration limit control, a fluid is supplied to or discharged from the cylinder mechanism. If a detected vibration stroke is a contracting stroke, the cylinder mechanism is controlled so as to be extended. If a detected stroke is an extending stroke, the cylinder mechanism is controlled so as to be contracted. In this manner, a change in the attitude of the vehicle body is cancelled.
FIG. 4 shows an ordinary motor vehicle suspension control system such as that disclosed in Japanese Utility Model Laid-Open No. 62-181413.
As shown in FIG. 4, there are four suspension units: a suspension unit 10 for a right front wheel mounted on a vehicle body (hereinafter referred to as S.sub.FR); a suspension unit 11 for a left front wheel on the vehicle body (S.sub.FL); a suspension unit 12 for a right rear wheel on the vehicle body (S.sub.RR), and a suspension unit 13 for a left rear wheel on the vehicle body (S.sub.RL).
Each of the S.sub.FR 10, S.sub.FL 11, S.sub.RR 12, and S.sub.RL 13 is formed of a cylinder mechanism which includes a pneumatic suspension chamber and a shock absorber (both not shown), and is interposed between the vehicle body and the corresponding wheel.
Solenoid valves 14 to 23 serve for changeover control of communicated states of pipings. The solenoid valves 14, 15, 18, and 19 are opening-closing valves, while the solenoid valves 16, 17 and 20 to 23 are three-way changeover valves. Each of the three-way changeover valves 16, 17, and 20 to 23 includes a supply valve and a discharge valve, and these changeover valves form, together with the pipings, supply means and discharge means for pneumatic suspension chambers of the S.sub.FR 10, S.sub.FL 11, S.sub.RR 12, and S.sub.RL 13.
First pipings from the solenoid valves 20 to 23 are independently connected to the S.sub.FR 10, S.sub.FL 11, S.sub.RR 12, and S.sub.RL 13. A first piping of the solenoid valve 16 communicates with second pipings of the solenoid valves 20 and 21. A first piping of the solenoid valve 18 communicates with third pipings of the solenoid valves 20 and 21. A first piping of the solenoid valve 19 communicates with third pipings of the solenoid valves 22 and 23. A first piping of the solenoid valve 15 communicates with second pipings of the solenoid valves 18 and 19.
A reserve tank 24 has a high pressure chamber which communicates with a second piping of the solenoid valve 15. A compressor 25 is controlled by opening/closing of the solenoid valve 14. A drier 26 is connected to an output piping from the compressor 25.
The drier 26 communicates with the high pressure chamber of the reserve tank 24 through a check valve in a normal direction, and also communicates with second pipings of the solenoid valves 16 and 17 through a check valve in a reverse direction.
Third pipings of the solenoid valves 16 and 17 communicate with a low pressure chamber of the reserve tank 24.
A pressure sensor 27 detects a pressure PL of the pressure chamber of the reserve tank 24. A pressure sensor 28 detects a pressure PH of the high pressure chamber of the reserve tank 24, and a pressure sensor 29 detects the pressure in the first piping of the solenoid valve 19 as a suspension pressure PS. An acceleration sensor 30 detects an acceleration G in the vertical direction of the vehicle body. A vehicle height sensor 31 detects a height HF of a front portion of the vehicle body. A vehicle height sensor 32 detects a height HR of a rear portion of the vehicle body. A vehicle speed sensor 33 detects a vehicle speed V, A steering sensor 34 detects a steering angle .theta..
Actuators 35 to 38 serve to mechanically change the damping forces of the shock absorbers and are provided in correspondence with the S.sub.FR 10, S.sub.FL 11, S.sub.RR 12, and S.sub.RL 13.
A control unit 40 controls, for suspension control, the solenoid valves 14 to 23, the actuators 35 to 38 and other components on the basis of detection signals from the sensors 27 to 34 and other components.
FIG. 5 is a flowchart of the suspension control operation of the control unit 40, showing a processing for detecting and determining vertical vibrations of the vehicle body and attitude control processing in accordance with vertical vibrations.
The operation of the conventional suspension control system will be described with reference to FIGS. 4 and 5. The processing shown in FIG. 5 is executed every period of predetermined sampling time (E.g., 6 m sec).
First, the solenoid valve 14 makes the compressor 25 effective under the control of the control unit 40 to supply air compressed by the compressor 25 to the high pressure chamber of the reserve tank 24 through the drier 26.
Also, the solenoid valves 15 to 23 in association with the S.sub.FR 10, S.sub.FL 11, S.sub.RR 12, and S.sub.RL 13 operate under the control of the control unit 40 to supply compressed air accumulated in the high pressure chamber of the reserve tank 24 to the S.sub.FR 10, S.sub.FL 11, S.sub.RR 12, and S.sub.RL 13 and to release the compressed air in the S.sub.FR 10, S.sub.RR 12, and S.sub.RL 13 to the low pressure chamber of the reserve tank
At this time, the magnitude (amplitude) and the period of vertical vibrations of the vehicle body are measured on the basis of the acceleration G and air is supplied to or discharged from the pneumatic suspension chambers of the S.sub.FR 10, S.sub.FL 11, S.sub.RR 12, and S.sub.RL 13 in accordance with the amplitude and the period to reduce changes caused in the attitude of the vehicle.
For this operation, the solenoid valves 15 to 23 are controlled in accordance with the processing routine shown in FIG. 5, as described below.
First, the acceleration G detected by the acceleration sensor 30 is read as data on vibrations of the vehicle body in the vertical direction (step S1), and the period and the amplitude of vertical vibrations of the vehicle body are measured (step S2).
Next, determination is made as to whether the period of vertical vibrations of the vehicle body satisfies a condition that it is within a period range in which the attitude control is to be effected (step S3). If YES (the period condition is satisfied), determination is then made as to whether the amplitude of the vertical vibrations satisfies a control start condition (stem S4).
The control start condition used as determination criterion in step S4 is previously set as a first threshold value (determination criterion) in the control unit 40. Accordingly, if the amplitude of the vehicle vibrations is greater than the first threshold value, it is determined that the control start condition is satisfied.
If the result of determination in step S4 is YES (the control start condition is satisfied), determination is made as to whether or not a control for a contracted stroke of the vertical vibrations is to be performed (step S5).
If the result of determination in step S5 is YES contracted stroke control), the solenoids are controlled so that the cylinder mechanisms of the S.sub.FR 10, S.sub.FL 11, S.sub.RR 12, and SR.sub.13 are extended (step S6). Conversely, if NO (extending stroke control), the solenoids are controlled so that the cylinder mechanisms are contacted (step S7). Then, the process returns.
In this manner, a vibration amplitude limiting control is performed so that vibrations of the vehicle body are cancelled. Once the vibration limiting control in steps S6 and S7 described above is started, the control start determination criterion of step S4 is set to a value of a lower level (second threshold value) for a certain period of time in order to prevent chattering and to sufficiently reduce vertical vibrations.
That is, the second threshold value (lower limit value) smaller than the first threshold value is used as a comparison criterion in step S4 to ensure that the amplitude after the start of the control is determined as a value greater than the second threshold value, and that the limiting control in steps S6 and S7 is continued until vertical vibration are sufficiently converged.
Thereafter, if it is determined in step S4 that the vibration amplitude is smaller than the second threshold value or in step S3 that the vibration period is out of the predetermined range, the limiting control is terminated (step S8), and the process returns. At this time, the determination criterion of step S4 is reset to the first threshold value.
However, there is a possibility that vertical vibrations of the vehicle body will be continuously detected for a long time during, for example, continuous traveling on an expressway or the like under certain road surface conditions. In such a situation, if the vibration limiting control start condition (first threshold value) is set to a fixed value, the vibration limiting control is restarted always in response to vibrations of the same level, and a control operation of continuously supplying air to and discharging air from the cylinder mechanisms is repeated.
Accordingly, the difference between the pressures in the high pressure chamber and the low pressure chamber of the reserve tank 24 is reduced and the difference between the pressures in the fluid suspension chambers of the S.sub.FR 10, S.sub.FL 11, S.sub.RR 12, and S.sub.RL 13 at the time of air supply and the pressures in these chambers at the time of air discharge becomes substantially zero. The effect of vibration limiting control is thereby reduced. Also, the operating frequency of the actuator elements including the solenoid valves 14 to 23 and the compressor 25 is increased, resulting in a reduction in the durability, i.e., the life of such elements.
As described above, since a start condition of vibration limiting control is set to a fixed value in the conventional suspension control system, the operation of supplying air to and discharging air from the fluid cylinder mechanisms in the S.sub.FR 10, S.sub.FL 11, S.sub.RR 12, and S.sub.RL 13 is performed repeatedly until the difference between the pressures during air supply and discharge is reduced so that the vibration limiting effect is considerably low, if vibrations are continuously detected. The problem of a reduction in the durability of the compressor 25 and other components is also encountered.